I have a mac-mini and a macbook wirelessly connected to an Airport Extreme (the new AC model). This Airport Extreme is connected to an external 1 TB portable hard disk. Both the computers are periodically backed up on schedule by using the Time Machine software. Yesterday I connected the macbook to the power supply (it is configured to backup not on batteries but only when connected to the mains power supply). The display mentioned 10 gb to backup but the available space was only 12 GB. I checked today morning and it was not backed up. Time Machine also always displays that the oldest backups are deleted for making space for new backups. Which means, I assume, that the backups will eternally take place even with the 1 TB size limitation. Therefore I fail to understand as to why the latest backup did not take place.